Self-aligned liner method of avoiding pl gate damage

ABSTRACT

A self-align method of preparing semiconductor gates for formation of a silicide, such as a cobalt silicide (CoSi) layer, is disclosed. Deposition of silicon nitride (SiN) and low-temperature oxide (LTO) liner types, the SiN liner having an overhang structure, prevent damage to the gates while forming a self-aligned source. The undamaged gates are suitable for CoSi deposition.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.61/778,491 filed on Mar. 13, 2013 and entitled SELF-ALIGNED LINER METHODOF AVOIDING PL GATE DAMAGE, the entire contents of which are herebyincorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to semiconductor fabricationmethods and, more particularly, to a method of improving reliability ofscaled-down memory cells.

2. Description of Related Art

Increased miniaturization of components, high performance, and low costof integrated circuits have long been goals of the computer industry.One technique in the continuing scaling-down of semiconductor memorycells involves fabrication of a semiconductor gate, such as a controlgate of a flash memory cell, including depositing a metal-containingmaterial onto a semiconducting layer as a semiconductor gate to formsilicide. However, if and to the extent the gate is damaged duringfabrication, an associated deformed or non-uniform profile of the gatecan adversely affect the silicide formation and, hence, chip quality.

A situation may arise, for example, where damage to a gate, such as apolysilicon (PL) gate, may cause, or contribute to, or occur inconnection with, a following silicide formation being too closelydisposed to an insulating layer. When the insulation layer is a chargetrapping dielectric of the gate, such as an oxide-nitride-oxide (ONO)layer of a memory device, electrical problems may occur as a consequenceof the silicide being formed too closely to the insulation layer. Suchoccurrence may undesirably manifest, for instance, resistance,reliability, retention and disturb issues.

A need thus exists in the prior art for a method of forming undamaged oroptimized semiconductor gates suitable for silicide formation thereon,or the like, or improved operation therewith.

SUMMARY OF THE INVENTION

The present invention is directed to addressing these needs by providinga method of preparing semiconductor gates, such as polysilicon (PL)gates, for silicide formation or the like. An implementation of themethod may comprise providing a semiconductor structure that includes asubstrate with a plurality of first PL features disposed above thesubstrate and insulated therefrom by an insulator/dielectric layer. Fora memory cell, with the semiconductor gates, e.g., PL gates, normallyspaced apart, a plurality of second semiconductor features, e.g., PLfeatures, may be disposed above the first PL features with the first andsecond PL features being insulated from one another. The implementation,further, may comprise disposing a first liner layer over thesemiconductor structure and disposing a second liner layer over thefirst liner layer. The second liner layer can be constructed with afirst width at an upper portion of the semiconductor structure and witha second width at a lower portion of the semiconductor structure, thesecond width being less than the first width.

According to a particular implementation, the disposing of the first andsecond liner layers may comprise disposing the second liner layer with afirst thickness at an upper portion of the second PL gates and with asecond thickness at a lower portion between spaced-apart first PL gates,the second thickness being less than the first thickness. The disposingof the second liner layer, according to another implementation, maycomprise creating an overhang near an upper portion of the second PLgates, thereby facilitating formation of a self-aligned source in thesubstrate.

While the apparatus and method has or will be described for the sake ofgrammatical fluidity with functional explanations, it is to be expresslyunderstood that the claims, unless indicated otherwise, are not to beconstrued as limited in any way by the construction of “means” or“steps” limitations, but are to be accorded the full scope of themeaning and equivalents of the definition provided by the claims underthe judicial doctrine of equivalents.

Any feature or combination of features described or referenced hereinare included within the scope of the present invention provided that thefeatures included in any such combination are not mutually inconsistentas will be apparent from the context, this specification, and theknowledge of one skilled in the art. In addition, any feature orcombination of features described or referenced may be specificallyexcluded from any embodiment of the present invention. For purposes ofsummarizing the present invention, certain aspects, advantages and novelfeatures of the present invention are described or referenced. Ofcourse, it is to be understood that not necessarily all such aspects,advantages or features will be embodied in any particular implementationof the present invention. Additional advantages and aspects of thepresent invention are apparent in the following detailed description andclaims that follow.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a cross-sectional diagram of a prior art semiconductor stackillustrating damage to a semiconductor, e.g., polysilicon (PL), gate;

FIG. 2A is a cross-sectional diagram of a semiconductor stackillustrating an intermediate step in a process of fabricating a PL gatethat includes layers of silicon nitride (SiN) and low temperature oxide(LTO) viewed along a Y-axis;

FIG. 2B is a diagram illustrating a view of the structure of FIG. 2Ataken in a direction of an X-axis;

FIG. 3A is a cross-sectional diagram showing an effect of depositing anorganic dielectric layer (ODL), a silicon-containing hard mask bottomanti-reflecting coating (SHB) layer, and a layer of patternedphotoresist (PR) onto the semiconductor stack of FIG. 2A as viewed alonga Y-axis;

FIG. 3B is an X-axis view of the formation of FIG. 3A;

FIG. 4A illustrates a result of performing a self-aligned source (SAS)etch procedure on the structure of FIG. 3A;

FIG. 4B depicts an effect of removing remaining ODL material from thedevice of FIG. 4A;

FIG. 4C shows the structure of FIG. 4B with the SiN layer removed;

FIG. 4D a illustrates an effect of removing the LTO layer from thestructure of FIG. 4C to expose PL gates as viewed along a Y-axis;

FIG. 4D b is an X-axis view of the structure of FIG. 4D a;

FIG. 4E a describes a result of forming a silicide on the PL gates inthe formation of FIG. 4D a;

FIG. 4E b shows detail of a SAS region of the structure of FIG. 4E a;

FIG. 5 is a flow diagram summarizing an implementation of a method offabricating PL gates according to the present invention; and

FIG. 6 is a cross-sectional diagram of a semiconductor stack comprisingPL gates.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS

Embodiments of the invention are now described and illustrated in theaccompanying drawings, instances of which are to be interpreted to be toscale in some implementations while in other implementations, for eachinstance, not. In certain aspects, use of like or the same referencedesignators in the drawings and description refers to the same, similaror analogous components and/or elements, while according to otherimplementations the same use should not. According to certainimplementations, use of directional terms, such as, top, bottom, left,right, up, down, over, above, below, beneath, rear, and front, are to beconstrued literally, while in other implementations the same use shouldnot. The present invention may be practiced in conjunction with variousintegrated circuit fabrication and other techniques that areconventionally used in the art, and only so much of the commonlypracticed process steps are included herein as are necessary to providean understanding of the present invention. The present invention hasapplicability in the field of semiconductor devices and processes ingeneral. For illustrative purposes, however, the following descriptionpertains to a method of manufacture of semiconductor gates.

Referring more particularly to the drawings, FIG. 1 illustrates astructure 100 formed in an intermediate step of a semiconductorfabrication process according to the prior art. The illustratedstructure 100, which may be a portion of a large array formed byrepeated patterns of which the structure 100 of FIG. 1 isrepresentative, comprises a substrate 10, having a side on which areformed first features of a first silicon layer and second features of asecond silicon layer, the first and second silicon layers being spacedapart by an insulator layer, such as a dielectric layer. The first andsecond silicon layers may comprise one or more of polysilicon oramorphous silicon. According to the described arrangement, the first andsecond features comprise, respectively, first semiconductor gates andsecond semiconductor gates, e.g., first polysilicon (PL) gates 30 andsecond PL gates 40.

The first PL gates 30 and second PL gates 40 are separated and insulatedfrom each other by one or more insulating layers, such as chargetrapping dielectric layers, e.g., in the form of oxide-nitride-oxide(ONO) layers 50 of a memory device. The device may form a portion of,for example, a memory chip wherein a source region 41 (not shownexplicitly) in the substrate 10 may be formed in a region disposednominally between a relatively closely-spaced pair of PL gates 30/40,the source area being penetrated by a trench 65 as a result of aself-aligned source (SAS) etch. A drain site 42 (not explicitly shown)may occupy a region in the substrate 10 nominally between a relativelymore widely-spaced pair of PL gates 30/40.

As semiconductor cells are scaled, cobalt silicide (CoSi), which maysupport thinner line widths than other materials such as, for example,WSi or TiSi, may be formed as a cap on the second PL gates 40. Such acap may best be formed on an upper surface of the PL gates 40 when thesurface is flat as depicted by the dashed outline 60 in FIG. 1. Whenprior art methods are used, however, the etch step used to form thetrench 65 may damage, e.g., partially remove material of, the second PLgates, resulting in a gate profile having a rounded corner 61.

Forming a silicide layer, e.g., a CoSi layer, on the upper surface ofsecond PL gates 40 can be expected to result in a deformed, non-uniformCoSi profile 191 (shown on only one PL gate 40 for simplicity) caused bythe rounded corner 61. Such a deformed profile could lead to aresistance issue, such as from a damaged profile having an unsmoothand/or zigzag topography.

Further, a separation between the ONO layer 50 and a CoSi layer 191 onan upper surface of PL gate 40 may be desired to be, e.g., a distanceD₁, which may be a value ranging from about 500 Å to about 2000 Å asmeasured in a direction normal to the side of the substrate. However,due to the effect of the rounded corner 61, the actual distance maybecome, e.g., D₂, measured in the same direction, which, in some casesmay be smaller than 500 Å and which may lead to electrical problems suchas reliability, retention and disturb issues.

FIG. 2A is a cross-sectional diagram (viewed along a Y-axis) depicting asemiconductor structure 200 in an intermediate stage of a less damagingfabrication process according to the present invention. The illustratedstructure 200 comprises a substrate 10 that may be formed of silicon. Agate oxide layer 20 having a thickness (TOX) of about 80 Å is disposedover the substrate 10. A pair of PL gates 30 and 40 are stacked on theoxide layer 20, the PL gates 30 and 40 being separated (i.e., insulatedfrom each other) by an ONO layer 50 disposed between them. Formation ofthe gate oxide layer 20, PL gates 30, the ONO layer 50 and the PL gates40 may be accomplished using methods known in the art. For example,oxide material such as tunnel OX may be deposited on the substrate 10 toform the gate oxide layer 20, and a first layer of conductive materialsuch as metal or polysilicon may be deposited over the oxide layer 20.Layers of oxide, nitride, and oxide again may be deposited sequentiallyover the first layer to form an ONO layer, followed by a secondconductive layer being formed on the ONO layer. The resulting stack oflayers may be patterned and etched using photolithographic methods toform a pair of semiconductor gates, e.g., PL gates, 30 (from the firstconductive layer) and 40 (from the second conductive layer), gates 30and 40 being separated by a charge-trapping structure, e.g., ONO layers50, as illustrated in FIG. 2A.

According to an aspect of the present invention, the structures formedby the PL gates 30 and 40 and the ONO layer 50 may be overlaid with afirst liner formed by a thin layer of low temperature oxide (LTO) 70such as SiO₂ using a known process, for example, of deposition at atemperature ranging from about 50° C. to about 500° C. A typicalthickness of the LTO layer 70 may range from about 2 nm to about 50 nm.

Further, according to another aspect of the invention, the resultingstructure may have overlaid thereon a second liner comprising arelatively thick silicon nitride (SiN) layer 80 having distinctivefeatures. In particular, the SiN layer 80 may exhibit an overhangprofile 85 whereby an exposed portion of the SiN layer 80 has a width W1at an upper portion thereof, i.e., near upper surfaces of the second PLgates 40, that is greater than a width W2 at lower portions of the firstPL gates 30. Further, a thickness T1 of the upper portion of the SiNlayer 80 may be greater than a thickness T2 of a lower portion of theSiN layer 80 disposed directly above the gate oxide layer 20.

In representative embodiments, the width W1 may range from about 50 nmto about 200 nm, preferably about 100 nm, with the width W2 ranging fromabout 25 nm to about 50 nm with a preferred value of about 40 nm. Thethickness T1 may take values between about 100 Å and about 1000 Å; thethickness T2 may range from about 50 Å to about 500 Å. Respectivepreferred values for T1 and T2 are about 500 Å and 80 Å. A typicalthickness of the SiN layer 80, measured at the lower portion and in adirection parallel to the side of the substrate 10, may range from about50 Å to about 500 Å (or to about 750 Å in other embodiments).

One method of forming the thick SiN layer 80 employs a plasma-enhancedchemical vapor deposition (PECVD) process. Using SiH₄+N₂O in a PECVDtool and then adjusting high SiH₄ and high bias power can produce anoverhang profile 85 in the SiN layer 80 as illustrated in FIG. 2A.

FIG. 2B illustrates a cross-section of the structure 200 of FIG. 2A asviewed along an X-axis. The figure, which illustrates the substrate 10,the gate oxide layer 20 and the SiN layer 80, also illustrates across-section of shallow trench isolation (STI) structures 90 that mayform a portion of the semiconductor structure 200.

Continuing, with reference to FIG. 3A, which is built upon the structure200 of FIG. 2A, an organic dielectric layer (ODL) 110 formed ofcarbon-like materials by, e.g., a spin coating process, may be overlaidon the SiN layer 80, the ODL layer 110 having a nominally planar surfaceand covering top portions of SiN layer 80 to a depth of about 500 Å. TheODL layer 110 may have disposed thereon a silicon-containing hard maskbottom anti-reflection coating (SHB) layer 120 formed by a spin coatingprocess and having a thickness of about 300 Å on which is formed a layerof patterned photoresist (PR) material 130. The PR, SHB, and ODL layers130, 120 and 110 may act to protect lower layers (e.g., a drain site 42disposed in the substrate and not explicitly shown) of the illustratedstructure during a subsequent etch step. The layers, further, canprovide high resistance to etching and be easily removed. A furtherpurpose of these three layers is using PR to open SHB (selectivity isabout 1) and then using SHB to open ODL (SHB has high selectivity toODL). An X-axis view of the structure 200 is illustrated in FIG. 3B.

FIG. 4A illustrates a result of a self-align (SiN layer) SAS etch havingbeen implemented to remove the SHB layer 120 and portions of the ODL 110from the structure of FIG. 3A according to the pattern of the PR layer130. The SAS etch may employ anisotropic etching gases such as C₄F₆,C₄F₈, C₅F₈, for example, having a high oxide selectivity with respect toSiN. In representative embodiments, a selectivity ratio of oxide to SiNmay range from about 3 to about 15, a preferred value being about 10.The high selectivity ratio of oxide to SiN not only can break throughthe oxide layer 20 but also can effectively protect the SiN layer 80thereby avoiding PL Gate damage. As used herein, a statement that anetch has a high selectivity to material A with respect to material Bmeans that the etch has a much stronger effect on A than on B with aresult that much more of A than B is removed when the etch acts on bothmaterials.

The SAS etch may remove a portion, but not all, of the SiN layer 80. Forinstance, the SAS etch may substantially eliminate or reduce theoverhang profile 85 (FIG. 2A) of the SiN layer 80, e.g., to a shape 88.The overhang profile 85 of the SiN layer 80 may help to achieve optimalformation/operation of a trench 140 (cf. below) by, e.g., effectivelycompensating for some inevitable partial removal of the SiN layer 80during the SAS etch, thereby protecting the PL gates 40 from damage of atype shown in FIG. 1. In typical embodiments, following the SAS etch, athickness of the SiN layer 88, measured at the lower portion and in adirection parallel to the side of the substrate 10, may remainunchanged, or alternatively, may remain about the same or only slightlychanged. To the extent moderately changed, the thickness may beattenuated so as to range, following the SAS etch, by way of exampleonly and not limitation, from about 25 Å to about 475 Å. According tomodified implementations, such as one in which the drawing isinterpreted to be to scale or about to scale, the thickness of the SiNlayer 88 can be about two to five times that of the LTO layer 70, eachof the thicknesses being measured at the lower portion and in adirection parallel to the side of the substrate 10.

When the SAS etch also forms the trench 140 in the silicon substrate 10,in line with an aspect of the invention, it is desirable that the trench140 resulting therefrom have nominally straight and vertical sides (cf.the T-shape profile illustrated in detail in FIG. 4E b). Further, thetrench 140 formed in the source region 41 of the substrate may berelatively narrow relative to the trench 65 (FIG. 1) that is formed whenprior art methods are used. Using the natural interpretation of thestructures formed by PL gates 30/40 and ONO layer 50 constituting firstand second memory cells, the trench will be disposed a first distancefrom the first memory cell and a second distance from the second memorycell, extend a depth into the substrate, and/or be characterized by awidth less than or about equal to either of (i) the first distanceand/or (ii) the second distance; according to one implementation thetrench has a width about equal to (iii) a sum of the first distance andthe second distance. One or more of (i), (ii) and (iii) may correspondto any one or more of the illustrations of FIGS. 4A-4E a, in which, forinstance, by way of example and not limitation, certaininstances/implementations of the embodiments can be interpreted to be toscale.

Hence, in certain implementations, either the first distance and/or thesecond distance may correspond to the sum of the thickness of the SiNlayer 88 and the LTO layer 70, each being measured at the lower portionand in a direction parallel to the side of the substrate 10.

According to some implementations, such as one in which the drawing isinterpreted to be to scale, the trench 140 is about as wide as, or inother implementations, narrower than, the thickness of the SiN layer 88at the lower portion as measured in a direction parallel to the side ofthe substrate 10. According to other implementations, such as ones inwhich the drawing is interpreted to be to scale yet again, the trench140 is about as wide as, or in other implementations, narrower than, asum of the thickness of the LTO layer 70 and the SiN layer 88 at thelower portion as measured in a direction parallel to the side of thesubstrate 10. Additionally, or alternatively, the trench 140 can beabout as wide as, or in other implementations, narrower than, its depth.

The T-shape profile illustrated in FIG. 4E a evidently corresponds torelatively less damage to the substrate 10 than results from applicationof prior art methods. SiN material not removed by/during the SAS etchprocess may form a hard mask that causes the SAS etch to form thedesired relatively narrow trench 140. It may also be noted that,although the SAS etch step may remove a portion of the SiN layer 80, anupper part of the SiN layer 80 is not removed while the lower part, thepart directly disposed above the oxide layer 20, is removed. Thisselective removal is a consequence of forming the SiN layer 80 withT1>T2.

The PR layer 130, which may function as a hard mask for the SAS etchstep, may subsequently be removed by a process involving, e.g., an ODLetch.

The ODL 110 may then be stripped from the structure using a dry stripprocess, causing the structure of FIG. 4A to appear as shown in FIG. 4B.The SiN layer 80 then may be removed by, e.g., a dry etch or a wet etch(using, e.g., hot phosphoric acid) having a high SiN to oxideselectivity and a high selectivity of SiN to PL leaving, as illustratedin FIG. 4C, PL gates 30 and 40 intact under an overlying LTO layer 70.The LTO layer 70 may act as a stop layer for this SiN etch, therebyprotecting the ONO layer 50 from damage during the SiN etch. The SiNetch may employ a selective etch gas such as, for example, CH₃F, CHF₃and CH₂F₂. Hot phosphoric acid (H₃PO₄) can be used in micro-fabricationto etch the SiN, and such can provide high selectivity to oxide. Andusing C₄F₈, C₄F₆, C₅F₈ (more carbon) etching gas can obtain highoxide-to-PL selectivity. Illustrative values for SiN-to-oxide ratios arebetween about 1 and about 25 with a ratio of about 5 being preferred.

The LTO layer 70 may subsequently be removed using an etch having highoxide-to-PL selectivity (e.g., a dry etch or SiConi™ etch) to expose thePL gates 30/40 as illustrated in FIG. 4D a. Anisotropic etching gasessuch as, for example, CF₄, CHF₃, CH₃F, CH₂F₂, C₄F₆, C₄F₈ and C₅F₈, andisotropic etching gases that may include, e.g., NH₃ and HF₃, may exhibita high degree of selectivity of oxide to PL. According to a preferredembodiment, isotropic etching gases are used for commensurate isotropicetching that can avoid ion-bombardment damage to the PL Gates.By-product exhaust gas corresponding to these etching gases may contain,for example, H₂, CH₄, H₂O and NH₃ when gas temperatures exceed 100° C.In these higher temperature conditions (>100° C.), C, O, N residues online pattern surfaces will be evaporated easily in the highertemperature condition. As illustrated in FIG. 4D a, and with referenceto the descriptions of FIGS. 2A and 4A, the SiN layer 80 may protect theLTO layer 70 and, consequently and more importantly, the PL gates 40from effects of the SAS etch that forms the trench 140. The PL gatestructure 40 therefore may be substantially unaffected by the SAS etchstep. Subsequent process steps (i.e., removal of the SiN layer 80 andthe LTO layer 70) may expose the PL gates 30/40 in an undamaged form.That is, an upper surface 160 of PL gate 40 may be nominally flat incontrast with the profile of PL gate 40 that results when prior artmethods are used (cf. rounded corner 61 in FIG. 1). As a consequence ofthe invention, the undamaged PL gate 40 is thus better able to haveformed thereon a silicide, e.g., a layer of CoSi. FIG. 4D b illustratesa view of the structure 200 along an X-axis.

Formation of a silicide, e.g., a CoSi layer 190, on the PL gates 40,illustrated in FIG. 4E a, may be accomplished by a rapid thermalannealing (RTA) process. Here, as a consequence of damage to corners(cf. 60, FIG. 1) of the PL gates 40 being attenuated or eliminated,corners are not disadvantageously deformed, e.g., rounded, as was thecase with corners 61 of FIG. 1. The silicide, e.g., CoSi layer 190exhibits a relatively uniform shape and optimized function, with allregions of the silicide layer in the depicted embodiment being spacedsufficiently from the ONO layer 50. Hence, the silicide formation is nottoo closely disposed to the insulating layer, e.g., the ONO layer 50,whereby resistance, reliability, retention and/or disturb issuesadvantageously can be reduced or avoided. By way of comparison withreference to FIG. 1, according to a feature of the invention, all of theCoSi layer is formed at a distance corresponding to D1 rather than D2.

As further consequence of the invention, a recess profile in the siliconsubstrate 10 in the source region 41 between PL gates 30 may have aT-shape as depicted in FIG. 4E b. This T-shape, obtained by employing anetch having high oxide, nitride to PL selectivity and by using two typesof liner (SiN and LTO), may be contrasted with the relatively wideprofile of the trench 65 (FIG. 1) obtained using prior art methods. ThisT-shape can be advantageous, owing, for example, to less siliconsubstrate damage being produced.

An aspect of the present invention is a method of forming PL gateshaving a profile suitable for deposition of CoSi thereon. FIG. 5 is aflow diagram summarizing one implementation of the method. Theimplementation commences at step 300 by providing, with reference toFIG. 6, a semiconductor structure 200 corresponding to an earlierprocessing stage of FIG. 1, or any of the other figures (or otherapplications/contexts altogether), and comprising a substrate 10, whichmay be formed of silicon, a stack of first PL gates 30 and second PLgates 40, with second PL gates 40 separated by ONO layers 50 from thefirst PL gates 30. The first PL gates 30 may be insulated from thesubstrate by an oxide layer 20. First and second liner layers may besequentially deposited on the structure of FIG. 6 as follows (cf. FIGS.2A and 2B). The first liner, e.g., a layer of LTO, may be formed, e.g.,deposited, over the semiconductor structure 200 at step 305. At step 310the second liner, e.g., a SiN layer 80 may be formed over the LTO layer70. The SiN layer 80 may be disposed with a first thickness T1 above thePL gates 40 and with a second thickness T2<T1 above the substrate andbetween PL gates 30. The SiN layer 80, further, may be formed to bewider at an upper part than at a lower part thereof. That is, withreference to FIG. 2A, a portion of the SiN layer 80 near the top of PLgates 40 may have a first width W1 and a second width W2 near the bottomof PL gates 30 with W2<W1. The effect is to create an overhang profile85 in the upper portion of the SiN layer 80. As described above indiscussion relative to FIG. 2A, the overhang profile 85 may be obtainedby using SiH₄+N₂O in a PECVD tool and then adjusting high SiH₄ and highbias power to obtain the desired result.

Preparatory to performing an etch step to form a SAS, and with referenceto FIGS. 3A and 3B, an ODL layer 110 may be disposed over the SiN layer80 at step 315, and a SHB layer 120 may be formed on the ODL layer 110at step 320. At step 325, a patterned PR layer 130 may be formed on theSHB layer 120, the PR layer 130 being disposed to permit exposing aportion of the PL gates 30/40. Then, according to the patterned PR 130,a SAS etch may be performed at step 330 to remove the SHB layer 120 andportions of the ODL layer 110, portions of the SiN layer 80 and theoxide layer 20. A portion of the substrate 10 also may be removed by theSAS etch to form a trench 140, thereby creating the SAS. FIG. 4Aillustrates a result of performing the etch at step 330. The SAS etchstep may be performed using anisotropic etch gases such as C₄F₆, C₄F₈,C₅F₈, which exhibit high oxide-to-SiN selectivity, to remove materialbetween pairs of PL gates 30/40 not protected by the PR layer 130, whichmay function as a hard mask. Referring to FIGS. 2A and 4A, the etch mayoperate to remove the relatively thin (i.e., thickness=T2) portion ofthe SiN layer 80, portions of the relatively thick (i.e., thickness=T1)SiN layer 80 disposed above the PL gates 40, and portions of theoverhang profile 85. Because the thickness of the SiN layer 80 isgreater at the top than the bottom (i.e., T1>T2), SiN material remainsafter the SAS etch so that the PL gates 40 are protected from damage.Additionally, the SAS etch removes portions of the LTO layer 70, theoxide layer 20, and the oxide disposed in STI structures 90, which oxideneeds to be etched cleanly as illustrated, e.g., in FIG. 4D b, therebyforming the trench 140 illustrated in FIG. 4A. As a consequence of theaction of the SAS etch step, sides of the trench 140 (i.e., the SAS) maybe nominally straight, narrow and vertical.

The patterned PR layer 130 and the portion of the LTO layer 110 notalready removed may be removed at step 335. FIG. 4B illustrates theeffect of the removal, the PL gates 30/40 being covered by only the LTOlayer 70 and the SiN layer 80. The SiN layer 80 and the LTO layer 70 maybe removed at, respectively, steps 340 and 345 of the implementation ofFIG. 5 (cf. FIG. 4C with SiN 80 removed and FIG. 4D a with the LTO 70removed). Etch gases such as CH₃F, CHF₃, CH₂F₂ having a selectivity ofSiN to oxide and etch gases such as CF₄, CHF₃, CH₃F, CH₂F₂, C₄F₆, C₄F₈,C₅F₈ having a selectivity of oxide to PL may be employed at this step.

It is a feature of the invention that a method, an implementation ofwhich is summarized in FIG. 5, may create PL gates 40 having undamagedprofiles as shown in FIG. 4D a. By undamaged is meant that an uppersurface 160 of the PL gates 40 is nominally flat and suitable forformation of CoSi. Accordingly, at step 350 of the implementation, CoSi190 may be deposited on an upper surface of PL gates 40 as shown in FIG.4E a.

An embodiment of the present invention comprises a semiconductorstructure formed according to the method described herein and furthercomprises a layer of CoSi formed on an upper surface of semiconductorgates, e.g., PL gates 40 (FIG. 4E a). A length of a separation betweenthe CoSi layer 190 and the ONO layer 50 may be approximately equal to adistance D₃ over all the PL gates 40 in the semiconductor structure.Further, the length of a separation measured on a left side of a PL gate40 may be about equal to a corresponding distance measured on a rightside of the same gate.

In accordance with a feature of the invention, a semiconductor structurecan be provided, having a substrate, a first memory cell and a secondmemory cell formed above a side of the substrate, and further having aT-shape trench in the substrate between the first memory cell and thesecond memory cell. The trench can be disposed a first distance from thefirst memory cell and a second distance from the second memory cell,extend a depth into the substrate, and/or be characterized by one ormore of (i) a width less than or about equal to the first distance, (ii)a width less than or about equal to the second distance, and (iii) awidth less than or about equal to a sum of the first distance and thesecond distance. Here, the T-shape can be conceptualized as comprising acenter with opposing arms and a stem extending outwardly therefrom, thearms being oriented to extend away from the center in directionsparallel to the side and corresponding geometrically to the firstdistance and the second distance, and the stem being oriented to extendin a direction transverse to the side into the substrate andcorresponding geometrically to the trench, whereby a T-shape resultsfrom the relative shapes and orientations of the first distance, thesecond distance and the trench. In contrast to the geometry depicted inFIG. 1, sidewalls of the trench can be vertical (or, alternatively,about vertical) to the side of the substrate and/or have more than aquarter, and, typically, more than half (or, alternatively, more thanthree quarters), of their lengths extending normal (or, alternatively,substantially normal) to the side. Furthermore, either or each of thefirst distance and the second distance can be defined by the sum of thethicknesses of the first and second liners.

Although the disclosure herein refers to certain illustratedembodiments, it is to be understood that these embodiments have beenpresented by way of example rather than limitation. The intentaccompanying this disclosure is to have such embodiments construed inconjunction with the knowledge of one skilled in the art to cover allmodifications, variations, combinations, permutations, omissions,substitutions, alternatives, and equivalents of the embodiments, to theextent not mutually exclusive, as may fall within the spirit and scopeof the invention as limited only by the appended claims.

What is claimed is:
 1. A method, comprising: providing a semiconductorstructure comprising: a substrate; a plurality of first features of afirst silicon layer disposed above a side of the substrate and insulatedtherefrom by an insulator layer; and a plurality of second features of asecond silicon layer disposed above the first features, the secondfeatures being insulated from the first features; disposing a firstliner layer over the semiconductor structure; and disposing a secondliner layer over the first liner layer, whereby the second liner layeris constructed with a first width at an upper portion of the secondfeatures and with a second width at a lower portion of the firstfeatures, the first and second widths being measured in a directionparallel to the side and the second width being less than the firstwidth.
 2. The method as set forth in claim 1, wherein the first andsecond features comprise first and second polysilicon (PL) gates, andthe disposing further comprises disposing the second liner layer with afirst thickness at an upper portion of the second PL gates and with asecond thickness at a lower portion between spaced apart first PL gates,the second thickness being measured in a direction normal to the sideand less than the first thickness.
 3. The method as set forth in claim1, wherein the first and second features comprise first and secondpolysilicon (PL) gates, and the disposing of the second liner layercomprises creating an overhang near an upper portion of the second PLgates.
 4. The method as set forth in claim 1, wherein the disposingcomprises: disposing a first liner layer composed of low-temperatureoxide (LTO) over the semiconductor structure; disposing a second linerlayer composed of silicon nitride over the first liner layer; disposingan organic dielectric layer (ODL) over the second liner layer; disposinga silicon-containing hard mask bottom anti-reflecting (SHB) layer overthe ODL; and disposing a patterned photoresist layer over the SHB layer,the patterned photoresist layer being disposed to expose a portion ofthe ODL and underlying layers.
 5. The method as set forth in claim 4,further including performing a self-aligned source (SAS) etch comprisingremoving portions of the ODL layer, the silicon nitride layer, the LTOlayer and the substrate according to the patterned photoresist layer andremoving oxide disposed in shallow-trench isolation structures betweenPL gates.
 6. The method as set forth in claim 5, further including:removing the second liner layer; removing the first liner layer; andforming a cobalt silicide layer on an upper surface of the secondfeatures.
 7. A semiconductor structure formed according to the method ofclaim 6, in which a length of separation between all parts of the cobaltsilicide layer and the insulator layer is approximately equal over theplurality of second features.
 8. The method as set forth in claim 5,wherein the removing employs etch gases having an oxide selectivityratio with respect to silicon nitride that ranges from about 3 to about15.
 9. The method as set forth in claim 5, wherein the first and secondfeatures comprise first and second polysilicon (PL) gates, and theremoving employs etch gases having a selectivity ratio of oxide tomaterial used to form the PL gates taking values greater than about 1,and wherein the method further comprises forming a cobalt silicide layeron an upper surface of the second PL gates.
 10. The method as set forthin claim 9, wherein the removing results in a T-shaped profile in thesubstrate between spaced-apart first PL gates.
 11. The method as setforth in claim 1, wherein: the first and second silicon layers compriseone or more of polysilicon or amorphous silicon; and the first andsecond silicon layers are insulated by a charge-trapping dielectriclayer.
 12. The method as set forth in claim 11, wherein: thecharge-trapping dielectric layer comprises an oxide-nitride-oxide (ONO)layer; the first liner comprises a relatively-thin low temperature oxide(LTO); and the second liner layer comprises a relatively-thick siliconnitride (SiN) layer.
 13. The method as set forth in claim 1, wherein thefirst and second features comprise first and second polysilicon (PL)gates.
 14. A semiconductor structure formed according to the method ofclaim
 2. 15. A method, comprising: providing a semiconductor structurecomprising: a plurality of first features of a first silicon layerdisposed above a side of a substrate and insulated therefrom by aninsulator layer; and a plurality of second features of a second siliconlayer disposed above the first features, the second features beinginsulated from the first features; and disposing a first liner layerover the semiconductor structure, and disposing a second liner layerover the first liner layer, whereby the second liner layer isconstructed with a first thickness on tops of the second features andwith a second thickness at bottoms of and between the first features,the first and second thicknesses being measured in a direction normal tothe side and the second thickness being less than the first thickness.16. The method as set forth in claim 15, wherein: the first and secondsilicon layers comprise one or more of polysilicon or amorphous silicon;and the first and second silicon layers are insulated by acharge-trapping dielectric layer.
 17. The method as set forth in claim16, wherein the charge-trapping dielectric layer comprises: anoxide-nitride-oxide (ONO) layer; the first liner layer comprises arelatively-thin low temperature oxide (LTO); the second liner layercomprises a relatively thick silicon nitride (SiN) layer; and the firstand second features comprise first and second semiconductor gates.
 18. Asemiconductor structure, comprising: a substrate; a first memory celland a second memory cell formed above a side of the substrate; and aT-shape trench in the substrate between the first memory cell and thesecond memory cell; wherein the trench is disposed a first distance fromthe first memory cell and a second distance from the second memory cell,extends a depth into the substrate, and is characterized by one or moreof (i) a width less than or about equal to the first distance, (ii) awidth less than or about equal to the second distance, and (iii) a widthless than or about equal to a sum of the first distance and the seconddistance, wherein the T-shape comprises a center with opposing arms anda stem extending outwardly therefrom, the arms extending away from thecenter in directions parallel to the side and correspondinggeometrically to the first distance and the second distance, and thestem extending in a direction transverse to the side into the substrateand corresponding geometrically to the trench, whereby a T-shape resultsfrom the relative shapes and orientations of the first distance, thesecond distance and the trench.
 19. The semiconductor structure as setforth in claim 18, wherein sidewalls of the trench are substantiallyvertical and oriented so as to extend in directions substantiallytransverse to the side.
 20. The semiconductor structure as set forth inclaim 18, wherein the trench is formed using a first liner layer havinga first thickness and a second liner layer having a second thickness,and one or more of the first distance and the second distance is definedby a sum of the first and second thicknesses.